This invention relates to processes and apparatus for producing piezoelectric polymers in the form of single or multi-layer films or tubular products having a central wire-type core by means of die drawing with simultaneous electrical poling. The obtaining of piezoelectric properties devolves from the existence of permanent dipolar orientation which does not exist when the polymer material crystallizes from the molten state.
Typical among the polymeric materials which are able to acquire piezoelectric properties are: polyvinylchloride (PVC), polyvinylfluoride (PVF), polyvinylidene fluoride and its copolymers with polytetrafluoroethylene, polytrifluoroethylene, polyvinylfluoride and mixtures using polymethylmethacrylate.
Typical prior art processes for preparing piezoelectric polymeric films are such as disclosed in U.S. Pat. No. 4,427,609 to Broussoux et al. that issued Jan. 24, 1984.
Preparation of piezoelectric tubular products having a wire core is disclosed in the paper published by Pantellis of British Telecom Research Laboratories. In the Pantellis process a polymeric tubular blank having a central wire core is drawn between two sets of draw rolls having a heated neckdown region therebetween whereby the polymer draw takes place in an extremely narrow neckdown region.
In applicant's invention the tubular blank having a wire core is pulled through a tapered die and much higher levels of piezo activity can be created because the drawing is taking place throughout the length of the interior of the tapered die chamber and the polymeric material throughout the entire die chamber can be exposed to an electric field to provide far higher piezo activity than with the heated neckdown technique of Pantellis.
In the Broussoux method of U.S. Pat. No. 4,427,609, the polymer blank orientation must be in the alpha phase initially and the drawing takes place between the nip of opposed crushing rolls. However, with the process of the present invention the initial polymer blank can be in any phase, alpha, beta, or gamma, or any combination thereof, and a useful piezo product provided.